Highly effective removal of antibiotics from aqueous solution by magnetic ZnFe₂O₄/activated carbon composite

In recent years, quinolones have attracted extensive attention for their application in improving human health (Zhao et al. 2017). As two typical fourth-generation quinolone antibiotics (Blondeau & Tillotson 2008; Hammama et al. 2018), gemifioxacin mesylate (GEM) and moxifloxacin hydrochloride (MOX) were widely used in the treatment of acute sinusitis, genital tract infection, respiratory tract infection (Cheng et al. 2003; Hammama et al. 2018), and other diseases for their curative effect and slight side effects. However, excessive use of antibiotics resulted in them entering the water and soil, causing a serious threat to humans due to the negative effects on the immune system. Therefore, effective removal of antibiotics from aqueous solution has become an urgent problem.

In the past decades, several methods were developed to remove these pollutants, for example chlorination (Huber et al. 2005) and membrane filtration (Snyder et al. 2007) methods. However, these methods have the disadvantages of secondary pollution or limited application conditions. By comparison, the adsorption method has the advantages of easy operation, is low cost, highly efficiency, and does not produce highly toxic by-products (Kyzas et al. 2013). Accordingly, it has been considered as an effective method to remove pollutants from aqueous solution.

Activated carbon (AC), as a commonly used adsorbent, was used to treat industrial wastewater (Luo & Li 2019), but is not easily separated from the solution after adsorption during industrial operations. In this respect, magnetic materials were found to be of great potential. As a typical magnetic substance, zinc ferrite (ZnFe₂O₄) has attracted attention for its magnetic property (Yu et al. 2003; Konicki et al. 2017). In previous reports, AC was studied for its adsorption of organic pollutants, including antibiotics (Zhang et al. 2016; Fu et al. 2017; Ndagijimana et al. 2019). However, the magnetic composites of AC and ZnFe₂O₄ were rarely reported for removing antibiotics, especially for MOX and GEM. In this paper, ZnFe₂O₄ was prepared onto AC via microwave method to produce a novel composite, ZnFe₂O₄/AC. Based on characterization results, this composite was found to combine the porosity of AC and the magnetism of ZnFe₂O₄. As a result, ZnFe₂O₄/AC exhibited high adsorption capacities for the antibiotics (MOX and GEM). The drug-loaded adsorbent can also be quickly separated from aqueous solution by a magnetic field. The adsorption isotherm, adsorption kinetics, the effects of pH and coexisting salts were also studied.

Iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O, 99.99%), zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, 99%), ethylene glycol (C₂H₆O₂, >99%), and AC were purchased from HWRK Chem. GEM and MOX were provided by Chengdu Sino-Strong Pharmaceutical Co. Ltd. The properties and molecular structures of the drugs are shown in Table 1.

ZnFe₂O₄/AC magnetic composite was prepared using a microwave-assisted hydrothermal method. In a beaker, Fe(NO₃)₃·9H₂O (8.08 g), Zn(NO₃)₂·6H₂O (2.98 g), and ethylene glycol (80 mL) were added and mixed for 20 min. Then, NaOH (0.5 M) solution was used to adjust the pH to 11. A light green transparent gel was obtained. Then AC (2.17 g) was added and the suspension stirred vigorously for another 20 min. The black mixture was transferred into a reaction still and reacted in a solvothermal microwave reactor (XH-800S-10) at 453 K for 20 min. After being cooled down to room temperature, the collected solid was washed with deionized water and ethanol to remove the residual reactants. Finally, the solid was dried at 373 K for 24 h.

ZnFe₂O₄ was prepared according to the above method without addition of AC.

The magnetism of the sample was characterized with a VSM-Versalab vibrating sample magnetometer. The Fourier transform infrared spectroscopy (FT-IR) spectroscopy data were recorded on a Nicolet iS50 FT-IR spectrometer. The phase composition of the sample was determined with a D8 Advance X diffractometer equipped with Cu Kα radiation (λ = 1.54178 Å). Nitrogen adsorption–desorption measurements at 77 K were performed on an AutosorbiQ-MP surface area analyzer. The zeta potentials data were obtained using a Zetasizer Nano ZS Zeta potential analyzer. The morphologies of the samples were determined with an FEI Inspect F50 field emission–scanning electron microscope (FE–SEM). The elemental analysis was carried on the SEM equipped with an energy dispersive X-ray (EDX) system.

The crystal structure of the sample was characterized with X-ray powder diffraction (XRD) measurements. Figure 1(a) shows that the main characteristic diffraction peaks of the synthesized ZnFe₂O₄ can be found at 29.96, 35.20, 36.98, 42.46, 53.34, 56.97, and 62.13°, respectively, corresponding to the crystal plane positions of 220, 311, 222, 400, 422, 511, and 440 of ZnFe₂O₄ recorded in the standard JCPDS database (Sharma et al. 2017), demonstrating the successful synthesis of ZnFe₂O₄. Furthermore, similar peaks can be found in the XRD pattern of ZnFe₂O₄/AC, indicating the composite contains ZnFe₂O₄ phase. On the other hand, the characteristic diffraction peaks of AC were not found for its amorphous structure. To verify the existence of AC in the composite, as well as the component of the sample, SEM–energy dispersive X-ray spectroscopy (EDS) characterization was carried out as shown in Figure 1(b). It can be seen that the composite consisted of the elements of C, O, Fe, and Zn. The molar ratio of Zn and Fe was ∼1:2, which is consistent with the standard ratio of ZnFe₂O₄. In relation to the high content of C, we therefore suggest the synthesized composite was composed of ZnFe₂O₄ and AC.

The morphology of ZnFe₂O₄/AC was characterized with SEM images. As shown in Figure 2, the composite consisted of the irregular micron-scale particles. ZnFe₂O₄ was dispersed on the surface of AC and the particle size ranged from 1 to 2 μm. Figure 3 shows the FT-IR spectra of ZnFe₂O₄, AC, and ZnFe₂O₄/AC. It can be seen that the signs in the spectrum of the synthesized ZnFe₂O₄/AC was almost consistent to those of AC because of the much higher content of AC in the composite, as demonstrated by the EDS data and SEM images. The peaks of ZnFe₂O₄/AC at 1,630 cm⁻¹ and 600 cm⁻¹ were attributed to C = O stretching (Chowdhury et al. 2012) and C–H bending vibrations (Sharma et al. 2017), respectively.

The porosity of the sample was verified by N₂ adsorption–desorption isotherm measurements at 77 K, as shown in Figure 4(a). The Brunauer–Emmett–Teller (BET) specific surface areas of ZnFe₂O₄ and AC were calculated to be 256.1 m² g⁻¹ and 1,223.6 m² g⁻¹, respectively, and the BET value of ZnFe₂O₄/AC was 723.5 m² g⁻¹, which is between AC and ZnFe₂O₄. In addition, the pore diameter distribution of the sample was calculated using the Barrett–Joyner–Halenda (BJH) method (Figure 4(b)). It was found that ZnFe₂O₄/AC was a mesoporous material with a pore diameter of ∼4 nm.

The magnetic property of the sample was also measured. As shown in Figure 5, the saturation magnetization of pure ZnFe₂O₄ was 50 emu g⁻¹. After being combined with AC, the magnetism reached 15.0 emu g⁻¹. From the inset of Figure 5, the composite in water can be easily controlled by an external magnetic field, indicating the convenient separation from liquid–solid systems.

Based on the characterization results, the synthesized magnetic ZnFe₂O₄/AC exhibited large BET specific surface area, large pore diameters, and excellent magnetism, indicating the possible potential in liquid-phase adsorption. Herein, the composite was used for adsorbing GEM and MOX with large sizes (14.46 Å × 7.91 Å and 8.60 Å × 14.23 Å) (Chai et al. 2019). First, the adsorption isotherms were measured to evaluate the adsorption capacity of ZnFe₂O₄/AC. The adsorption amounts of ZnFe₂O₄/AC for GEM and MOX increased with the increasing concentration, as shown in Figure 6. The experimental results showed that when the initial concentration of the solution was 1,000 mg L⁻¹, the adsorption capacity of ZnFe₂O₄/AC for GEM and MOX could reach up to 433.4 mg g⁻¹ and 388.8 mg g⁻¹, respectively.

To evaluate the performance of ZnFe₂O₄/AC, some comparisons were made with other materials. It can be seen that ZnFe₂O₄/AC exhibited better adsorption performance for GEM and MOX than other materials (Chai et al. 2019), including UiO-66, MIL-125-NH₂, MIL-53, MIL-101, and MOF-808. Although the capacities of the ZnFe₂O₄/AC are slightly lower than those of MIL-101-SO₃H, the magnetism allows this composite to be easily separated from the adsorbent-solution system.

The adsorption behavior of ZnFe₂O₄/AC was analyzed according to the Langmuir model and Freundlich model (Zhao et al. 2018).

Adsorption kinetics is an important factor to understand the adsorption reaction pathway (Gürses et al. 2014). As shown in Figure 8, fast adsorptions of GEM and MOX were found at only 5 min, and the adsorption capacities at this time can reach up to 70.2% and 67.9% of the saturated capacities, respectively. Adsorption equilibrium was achieved at ∼12 h for both GEM and MOX. The adsorption kinetics of GEM and MOX on ZnFe₂O₄/AC were further studied according to two classic kinetics models.

In general, solution pH has a large influence on ionic adsorbates upon electrostatic interaction. Thus, in this work, adsorption behaviors at pH 5–13 were studied systematically. The pH of the solution was adjusted using HCl (0.1 M) and NaOH (0.1 M) solutions. Figure 10(a) shows the adsorption capacities of GEM and MOX at pH 5–13. With the increase of solution pH from 5 to 13, the capacities first increased and then decreased gradually, and the optimal pH is 6–7.

To understand this phenomenon, the charge property of ZnFe₂O₄/AC was analyzed using zeta potential measurement at pH 5–13. As shown in Figure 10(b), ZnFe₂O₄/AC was positively charged at pH < 5.8, and negatively charged at pH > 5.8. On the other hand, according to the pKa values of these drugs (Table 1), GEM and MOX existed as the anionic forms at pH < 6.02 and pH < 6.43, respectively. Thus, at pH 6.0, electrostatic interaction may exist between surface negatively charged ZnFe₂O₄/AC and cationic GEM and MOX molecules, and the interactions may be weak. At pH ≤ 5.0, the surface of ZnFe₂O₄/AC shifted to be positively charged and, accordingly, the electrostatic interaction disappeared, leading to the reduced capacities. Similarly, at pH ≥ 7.0, the interaction did not exist between the surface of the negatively charged adsorbent and anionic GEM and MOX molecules.

Industrial wastewater commonly contains some coexisting inorganic ions, such as Na⁺, K⁺, Ca²⁺, Cl⁻, ⁠, and ⁠. Thus, from the practical viewpoint, it is necessary to explore the effect of these coexisting substances on the adsorption process. In this work, the concentrations of the salts were consistent with that of GEM and MOX (500 mg L⁻¹). From Figure 11, it can be seen that the coexisting substances, including monovalent and bivalent ions, have slightly negative effects on the adsorption of GEM and MOX. ZnFe₂O₄/AC may therefore have potential in the practical treatment of wastewater containing GEM and MOX.

ZnFe₂O₄ is commonly regarded as a photocatalyst (Li et al. 2011; Arimi et al. 2018; Liu et al. 2019). Therefore, to understand the removal mechanism of GEM and MOX, the effect of light on the adsorption was investigated, as shown in Figure 12(a) and 12(b). The concentration curves at visible light and dark environment almost overlapped completely, indicating photodegradation may not exist in the removal processes of GEM and MOX.

Furthermore, as stated in the previous section, electrostatic attraction interaction existed in the adsorption process of GEM and MOX at pH 6.0. However, a high adsorption capacity (>300 mg g⁻¹) was still obtained even with the electrostatic repulsion at pH ≤ 5.0 or pH ≥ 7.0. Thus, electrostatic interaction may not be the unique force for the adsorption of GEM and MOX on ZnFe₂O₄/AC. As shown in Table 1, GEM and MOX both contained some organic functional groups, such as –NH and –COOH, which can interact with C = O groups of AC via H-bond interaction. Previously, the adsorption of organic molecules with various groups over C-based materials has been explained by the formation of H-bond interaction (Baccar et al. 2012; Bhadra et al. 2016; Jauris et al. 2016). Therefore, we suggest that electrostatic interaction and H-bond interaction contributed to the adsorption of GEM and MOX on ZnFe₂O₄/AC.

ZnFe₂O₄/AC has been synthesized using the microwave method and studied for its adsorption performance toward GEM and MOX from aqueous solution. The largest adsorption capacity for GEM and MOX can reach 433.4 mg g⁻¹ and 388.8 mg g⁻¹, respectively. Adsorption data were well-fitted with the Freundlich isotherm and the pseudo-second-order model. The coexisting substances, including monovalent and bivalent ions, have slightly negative effects on the adsorption, and the ZnFe₂O₄/AC can be easily separated by its magnetism from water. The possible adsorption mechanism of GEM and MOX on ZnFe₂O₄/AC involves electrostatic interaction and H-bond interaction. Thus, this work may provide a guideline for improving GEM and MOX adsorption for future.

This work was supported by PhD Scientific Research Foundation of Taiyuan University of Science and Technology (No. 20182020) and Key Research Foundation of Science and Technology of Shanxi Province (No. 201803D121099).

The authors have no conflict of interests.

All relevant data are included in the paper or its Supplementary Information.